The invention concerns a process for the preparation of a cation-exchanging alkali metal aluminosilicate granulate and its use for the removal of cations causing hard water during washing and cleansing, especially during the washing of textiles and during the cleansing of dishes while the wash liquor is circulated.
A possible way of reducing the eutrophication of bodies of water by detergent phosphates is by using methods of washing and cleansing which do not employ detergent phosphates. This requires other methods of binding the cations causing hard water. In the German Published Applications (DOS) 25 43 946 and 25 56 827, for example, methods of washing and cleansing are disclosed where the wash solution is circulated and the cations causing hard water are removed by ion exchangers from the circulation wash liquors in these processes. The ion exchangers suitable for this purpose are among others, synthetically prepared alkali metal aluminosilicates, referred to as "Aluminosilicate" in the following text, which have a large capacity for the exchange of cations. Crystalline, granular aluminosilicates are more suitable than finely divided aluminosilicates because of their easier handling.
Granulated aluminosilicates have long been known and employed for purposes of adsorption or as carriers for catalysts. Both inert or active substances are used as binding agents for the formation of these granulates. If inert substances such as, for example, clays or amorphous silicic acid are used as binding agents, the granulates prepared in this manner have a lower adsorptive capacity because their aluminosilicate content is reduced by the proportion of binding agent, and consequently they also have a lower ion exchange capacity, which makes them less suitable for the purpose of ion exchange.
Aluminosilicate granulates that contain active binders are also known, however. For example, in the process for the preparation of the aluminosilicate forms described in the German Pat. No. 12 03 238, aluminosilicate is used as granulating and binding agent. To this end, an aluminosilicate granulate is prepared as a starting material. This granulate is obtained by mixing an aluminosilicate powder with an aqueous silicic acid sol or an aqueous alkali metal silicate solution to form a granulate which is then dried. The aluminosilicate binder is then formed by treating this granulate with an alkaline alkali metal aluminate solution. This treatment also contributes to the adsorptive capacity of the molecular sieve forms, while at the same time imparting a high resistance to mechanical wear to the product of the process. This latter property is desirable for its use as an adsorptive agent. This high resistance to mechanical wear, however, makes these products, which are designed for use as molecular sieves in separation equipment, unsuitable for use as ion exchangers in wash liquors. Their exchange rate is too low, and a partial decomposition into finely divided aluminosilicate occurs in contact with the wash liquor, which is heated to 30.degree. to 95.degree. C. If an adequate softening of the wash liquor is nevertheless to be achieved with the known, commercial aluminosilicate granulates when washing with hard water, the washing time would have to be increased, or a considerably larger amount of the then only incompletely utilized aluminosilicate granulate would have to be used. Furthermore, filters with fine pores would have to be built into the wash liquor circulating system, to prevent the adsorption of the fine particles produced by this partial decomposition on the wash. Such an expedient would cause a correspondingly great loss of pressure in the system, and would also increase the danger of clogging the filters with lint or dirt.
The disadvantages found in the products of the German Pat. No. 12 03 238 are also present in the products formed by the processes in the German Published Applications DAS 15 67 557 and DOS 27 07 313. These patents employ products which are obtained from crystalline zeolite with pulverized metakaolin as binder. The pulverized metakaolin binder is converted into zeolite by thermal treatment with sodium hydroxide. These products are also used as adsorptive agents.
The products of the German Published application DAS 15 67 566 and DOS 27 15 678, obtained by changing metakolin alone into zeolite by treating with sodium hydroxide and heat and extrusion into formed particles, also find application in adsorption equipment and possess the previously described disadvantages of such products as well. Products such as those described in the German Published Application DOS 11 95 730 are not suitable as ion exchangers for wash liquors for the same reason. The products described in this application are derived from pieces of kaolin, used as starting material and are converted into zeolite by a subsequent treatment, and combined with an aqueous solution containing oxides of silicon, aluminum and of an alkali metal.
The aluminosilicate granulate prepared according to the known processes of the state of technology possesses the additional disadvantage of a frequently very high alkali content in the binder, which is the cause of an often undesirable increase in the pH of the wash or cleansing solution brought in contact with the aluminosilicate granulate for the purpose of softening.
A further disadvantage of the process of the state of technology for the preparation of granulates having an aluminosilicate binder is the necessity to prepare the product of the process in several successive process steps. This lowers considerably the reproducibility of the products of the process, and an aluminosilicate with variable properties is obtained.